A sealant gun or grease gun of the prior art is a device activated by fingers pressing a trigger, whereby the disk on the tip of a push rod will be driven forward by leverage to eject a viscous material out of the gun. However, the conventional gun is inconvenient in that the pumping of the viscous material takes manual power and is discontinuous. Therefore, smooth ejection of the material is not possible. Sealant/grease guns powered by compressed air or electricity are invented to assure smooth and continuous ejection of the viscous material and to save manual power. However, there are many other problems, such as the hindrance of air hoses or power cords to the operation. It is a further problem in manual and electricity/compressed air powered sealant/grease guns that the viscous material will continue to drip out of a gun after the pumping stops because of the exertion of elastic force on the disk on the push rod by the deformed viscous material. During the pumping, the viscous material is squeezed so that potential energy is stored therein; and, since the push rod will not go quickly after the pumping stops, the restoring material will exert force on the rod. At the same time, the reaction of the disk on the rod will push a small amount of the material out of the gun, forming the problem of dripping. To prevent dripping, some manual guns are equipped with a release for pulling back push rods in time.
Each of the electric sealant guns disclosed by the patents U.S. Pat. No. 6,123,235 and TW00405447 has a conventional DC motors and a clutch, whereby the resistance against the drawback of push rod will be automatically released after a person stopping pushing the trigger, and whereby the viscous material left in the gun will be able to push the push rod backward. However, the dripping problem cannot be removed completely, because the disk at the end of the push rod still experiences resistant forces caused by the friction between various movable components associated with the clutch. Therefore, the viscous material still has to experience reaction from the disk, and the dripping cannot be fully stopped.